The present invention relates to a device for the preparation of hot drinks including at least two hot water boilers each having an electrical heating element, a temperature sensor, a cold water inlet, and an outlet. The device also includes a control mechanism with means for controlling electrical energy supplied to the heating elements depending on water temperatures measured in the boilers.
The invention relates in particular to a coffee machine which permits, in addition to the preparation of coffee, the preparation or heating up of other drinks, such as tea or milk, by providing hot water or steam.
There are coffee machines on the market in which two hot water boilers are built in. Each of the two contains a heating element to heat up the water stored therein and a temperature sensor to measure the water temperature. Each of the temperature sensors sends a signal to a control mechanism in which the heating element of the corresponding boiler switches on when the temperature drops below a set point, and switches off when the desired temperature is reached again.
While the first boiler is filled with hot water which is needed to prepare coffee, the second boiler is often only partially filled with hot water. In this case the filling amount is controlled by a level controlling device, and in the remaining volume of the second boiler there is steam. Cold water can be supplied to each of the boilers via a cold water inlet. The hot water of the first boiler can be drawn off via a hot water outlet, and can be conveyed to a brewing device to prepare coffee. The second boiler includes a steam outlet in the embodiment described above, which is disposed on the top of the boiler and which is provided additionally with a hot water outlet, foreseen below the hot water level. Coffee machine models are also available in which the second boiler is filled with hot water, and has only a hot water outlet, or where only steam, but no hot water, can be drawn off, as in the design presented above.
With these prior art coffee machines, each heating element, controlled via a corresponding switching means, is joined in parallel to the mains voltage. One assumes that at least as much power can be provided from the supply mains as the two heating elements together are able to turn into heat.
This is usually the case with three-phase supply mains. Normally each of the two heating elements is fed with a separate phase.
In the case of single-phase power mains and with existent power outlets, the problem arises that the available power, which is determined by the mains voltage and corresponding to the fuse value, lies below the maximum power necessary to supply the coffee machine when the two heating elements are switched on simultaneously. To avoid blowing a fuse, expensive rewiring has to be done in these cases even though the maximum power of the coffee machine is only rarely consumed. Namely only when, after switching on, the water in the boilers is being heated up, or only when warm water or steam is supplied just after coffee has been supplied, or vice-versa.